1. Field of the Invention
This invention relates to a reactive power control system, and more particularly to a reactive power control system using a plurality of cycloconverters.
2. Discussion of the Background
Reactive power compensation units and condenser units are generally known as units for improving the reactive power of power systems. However, a circulating current type cyclo-converter does not require these reactive power compensation units, and the reactive power can be controlled by controlling the circulating current independently of the load current of the cyclo-converter. In other words, the reactive power generation due to the load current when the load current is small is small, and the reactive power of the cyclo-converter itself can be controlled at a constant by passing a circulating current increased by the amount required. The power factor of the power system is always maintained at a high level and voltage fluctuation of the power system is cancelled out by installing a condenser unit having a leading capacity equivalent to the lagging reactive power generated by the cyclo-converter and using it to cancel out the lagging reactive power of the cyclo-converter. Thus, it is the circulating current type cyclo-converter which achieves a stable voltage power system without voltage fluctuation of the power system.
FIG. 1 shows an example in which this type of conventional circulating current type cyclo-converter is connected to a power system. When considering a rolling mill facility as the load connected to the cycloconverter, there are many cases in which 5 to 8 cyclo-converters and 3 to 5 condensers are provided.
In FIG. 1, n sets of load systems are connected to power source B. FIG. 1 shows only the first load unit and the nth load unit. In the first load system, controlled power is supplied to the load, for instance motor M1, from receiving point R of power system B via circuit-breaker CB1, transformer T1 and circulating current type cyclo-converter (hereinafter simply referred to as cyclo-converter) CONV1. In the same way, controlled power is supplied to motor Mn from power source B via circuit-breaker CBn, transformer Tn and cyclo-converter unit CONVn. Cyclo-converter units CONV1 and CONVn each consist of a cyclo-converter 10 directly connected to its respective load and the control unit (numerals 11 to 15) which controls it.
Also, m sets of condenser units are connected to power source B for compensating lagging reactive power of the load. Here also, only the first condenser unit and the mth condenser are shown. The first condenser unit is composed of circuit-breaker CBC1 and condenser SC1, and the mth condenser unit is composed of circuit-breaker CBCm and condenser SCm. Incidentally, a reactor is shown as connected in series with the condenser. This is used for adjusting the capacity of the condenser in the form of cancelling-out, and the two together are treated as the condenser.
The voltage at receiving point R is detected by voltage sensor PT1, and this detected voltage is conducted to each cyclo-converter unit CONV1 and CONVn. The load current of each load unit is detected by current sensors CT1 to CTn and is conducted to the controller of the cyclo-converter unit to which the current sensor concerned is dedicated. This controller is provided in each cyclo-converter unit, but FIG. 1 only shows the controller in first cyclo-converter unit CONV1. However, a controller with the same composition is provided in each cyclo-converter unit.
In cycloconverter unit CONV1, the reactive power is detected by reactive power detection device 11 based on the detected voltage and detected current which are conducted. The circulating current of cyclo-converter 10 is controlled by reactive power control device 12 based on the reactive power detected by reactive power detection device 11 via circulating current control device 13 and gate control device 15 so that it becomes a pre-set reactive power value. In the controller, load current control device 14 is provided independent of circulating current control device 13 for controlling the load current flowing in motor M1.
In the conventional circulating current type cyclo-converter unit described above, a condenser unit is provided which can supply leading reactive power equivalent to the maximum lagging reactive power generated by cyclo-converter 10. Therefore, when all the cyclo-converter units and all the condenser units are connected, the lagging reactive power of all the load units and the leading reactive power of all the condenser units cancel out and are neutralised so that no specific problems should arise.
However, in the operation of rolling facilities which are connected as the load, there are cases, called dummy rolling, when rolling is operated by shutting down at least one of the n rolling facilities depending on the rolling material, or cases of operation by separating-off a part of the rolling facilities due to the breakdown of cyclo-converters or condensers. In such cases, in a conventional cyclo-converter unit, the design is that the reactive power of each cyclo-converter is detected on the assumption that all loads are being operated, and each cyclo-converter is individually controlled based on the result of this detection. Therefore, when there is a variation in the load condition, such as a partial shutdown of the load as described above, optimal reactive power control cannot be performed. Thus, a problem will occur due to the increase of voltage fluctuation in the power system.
The above problem can be solved by altering the reactive power constant control settings of the cyclo-converters in each case according to the load conditions. However, not only does this require a great deal of work in practice, but also the shut-down times of the rolling facility become more frequent, thus leading to a fall in productivity.
In the case of the leading reactive power being too great, the transformers become over-excited due to the increase of the power voltage, and there is a risk of causing a temperature rise. Also, when the lagging reactive power is too great, the power voltage drops and, at the same time, the power factor also decreases. Therefore, there is an overload state due to the large power current flowing, and this becomes a cause of temperature rises in the cables.